1. Field of the Invention
The present invention relates to a display panel device, a display device, and a control method thereof, and particularly to a display panel device and a display device using current-driven luminescence elements, and a control method thereof.
2. Description of the Related Art
As image display devices using current-driven luminescence elements, image display devices using organic electroluminescence (EL) elements are known. The organic EL display devices using the organic EL elements, which are self-luminous, do not need a backlight that is necessary in the case of a liquid crystal display device. For this reason, such organic EL display devices are most suitable for manufacturing thinner devices. Moreover, having no limitation on the viewing angle, the organic EL display devices are expected to become commercially practical as next-generation display devices. In addition, the organic EL elements used in the organic EL display devices are different from liquid crystal cells in that luminance of each luminescence element is controlled according to a value of current applied to the luminescence element. Meanwhile, a liquid crystal cell is controlled according to a voltage applied.
In general, the organic EL display device includes the organic EL elements, which are pixels, arranged in a matrix. A display device referred to as a passive-matrix organic EL display device is explained as follows. An organic EL element is provided at each intersection point of row electrodes (scanning lines) and column electrodes (data lines). Then, a voltage corresponding to a data signal is applied between the electrodes of the selected row and the column electrodes, so that the organic EL elements are driven.
Also, a display device referred to as an active-matrix organic EL display device is explained as follows. A switching thin-film transistor (TFT: Thin Film Transistor) is provided at each intersection point of scanning lines and data lines. A gate of a driver is connected to the switching TFT. Through the selected scanning line, the switching TFT is turned ON and a data signal is fed from a signal line into the driver. By this driver, the organic EL element is driven.
In the case of the passive-matrix organic EL display device, only while the row electrodes (the scanning line) are selected, the organic EL elements connected to these row electrode produce luminescence. Unlike the passive-matrix organic EL display device, the active-matrix organic EL display device allows the organic EL elements to produce luminescence until a next scanning (selection). For this reason, an increase in the number of scanning lines does not result in a decrease in luminance of the display. Thus, the active-matrix organic EL display device can be driven at a low voltage, thereby achieving low power consumption. However, in the case of the active-matrix organic EL display device, due to variations in characteristics of driving transistors, even when the same signal is applied, luminance of the organic EL elements is different for each pixel, thereby causing a problem of variations in luminance.
In order to address this problem, Patent Literature 1 (Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2008-203657), for example, discloses a method of compensating for pixel-to-pixel variations in the characteristics using a simple pixel circuit, as the method of compensating for variations in luminance caused due to the characteristic variations of the driving transistors.
FIG. 14 is a diagram showing a circuit configuration of a pixel unit of a conventional display device disclosed in Patent Literature 1. A display device 500 shown in this diagram includes a pixel array unit 501, a horizontal selector 503, a light scanner 504, and a bias scanner 505. The pixel array unit 501 includes pixel units 502 arranged in a matrix in a plane.
The pixel unit 502 is configured with a simple circuit element which includes: a luminescence element 508 having a cathode that is connected to a negative power line 512; a driving transistor 507 having a drain that is connected to a positive power line 511 and a source that is connected to an anode of the luminescence element 508; a capacitor 509 connected between a gate and the source of the driving transistor 507; an auxiliary capacitor 510 connected between the source of the driving transistor 507 and a bias line BS; and a sampling transistor 506 having a gate that is connected to a scanning line WS, and selectively applying a video signal from a single line SL to the gate of the driving transistor 507.
The light scanner 504 supplies a control signal to the scanning line WS, and the horizontal selector 503 supplies a reference voltage Vref to the signal line SL. With this, a correction operation is performed whereby a voltage corresponding to a threshold voltage Vth of the driving transistor 507 is held in the capacitor 509. Then, following this, a writing operation is performed whereby a signal potential Vsig of the video signal is written to the capacitor 509.
Before the correction operation, the bias scanner 505 changes the potential of the bias line BS, and applies a coupling voltage to the source of the driving transistor 507 via the auxiliary capacitor 510. By doing so, the bias scanner 505 performs a preparatory operation whereby a voltage Vgs between the gate and the source of the driving transistor 507 is initialized to be higher than the threshold voltage Vth.
The pixel unit 502 negatively feeds the drain current of the driving transistor 507 back to the capacitor 509 in the operation of writing the signal voltage Vsig. With this, the signal voltage Vsig is corrected according to the mobility of the driving transistor 507.
FIG. 15 is an operation timing chart of the conventional display device disclosed in Patent Literature 1. This diagram shows an operation performed by the display device per pixel line, and shows that one frame period includes a non-luminescence period and a luminescence period. In the non-luminescence period, the correction operations are performed to correct the threshold voltage Vth and the mobility β of the driving transistor 507.
First, at a time T1 when the present frame period starts, a short control pulse is applied to the scanning line WS and the sampling transistor 506 is thus turned ON temporarily. Since the potential of the signal line SL is the reference voltage Vref at this time, this reference voltage is written to the gate electrode of the driving transistor 507. Then, Vgs of the driving transistor 507 becomes equal to or lower than Vth and, as a result, the driving transistor 507 is cut off. Accordingly, the luminescence element 508 stops producing luminescence and the display device 500 enters the non-luminescence period at the present time T1.
Next, at a time T2, a control signal pulse is applied to the scanning line WS so that the sampling transistor 506 is turned ON.
Immediately after this, at a time T3, the potential of the bias line BS is changed from a high potential to a low potential. As a result, the potential of the driving transistor 507 is lowered via the auxiliary capacitor 510. More specifically, a relationship between Vgs and Vth is expressed as Vgs>Vth, and the driving transistor 507 is thus turned ON. At this time, since the luminescence element 508 is reversely biased, the current does not flow and thus the source potential of the driving transistor 507 increases. Then, when Vgs=Vth, the driving transistor 507 is cut off and the threshold voltage correction operation is completed.
Following this, at a time T4, the potential of the signal line SL changes from the reference voltage Vref to the signal voltage Vsig. At this time, since the sampling transistor 506 is conducting, the gate potential of the driving transistor 507 is Vsig. Here, since the luminescence element 508 is in the cutoff state initially, a discharge current Ids which is the drain current of the driving transistor 507 flows only through the capacitor 509 where the electrical discharge accordingly starts. After this, by a time T5 at which the sampling transistor 506 is turned OFF, the source potential of the driving transistor 507 is increased by ΔV. In this way, the signal potential Vsig is written to the capacitor 509, being added to Vth, and at the same time, the voltage ΔV used for the mobility correction is subtracted from the voltage held in the capacitor 509. This period from the time T4 to the time T5 is a mobility correction period as well as a signal writing period. The higher Vsig, the larger the discharge current Ids and the larger an absolute value of ΔV.
FIG. 16 is a graph showing the characteristics of the discharge current of the capacitor in the mobility correction period. The horizontal axis denotes a lapse of time since the signal voltage Vsig is written, that is, a lapse of time after the time T4. The vertical axis denotes a value of the discharge current. When the gate potential of the driving transistor 507 is changed from the reference voltage Vref to the signal voltage Vsig at the time T4 as described above, the discharge current Ids makes a discharge curve, such as A1, B1, or C1, depending on the magnitude of Vsig. Here, A1 and A2 are discharge curves of the driving transistors in the case where the same magnitude of Vsig is applied to the gates of these driving transistors although these driving transistors have different characteristic parameters of the mobility β. Each of the relationships between B1 and B2 and between C1 and C2 is the same as the above-mentioned relationship between A1 and A2. It can be seen from these discharge curves that, even with the application of the same signal potential, initial values of the discharge current Ids are different when the characteristic parameters of the mobility β are different. However, the discharge currents Ids become almost equivalent to each other with the lapse of discharge time. For example, on comparison between A1 and A2, the discharge currents Ids become almost equivalent at a time a. On comparison between B1 and B2, the discharge currents Ids become almost equivalent at a time b. On comparison between C1 and C2, the discharge currents Ids become almost equivalent at a time c. To be more specific, even when the pixel array 501 includes the driving transistors having different characteristic parameters of the mobility β, the drain current of the driving transistor 507 is caused to be discharged, while the gate bias is applied such that the luminescence element 508 does not produce luminescence in the above-mentioned mobility correction period. Accordingly, the correction can be made, with consideration given to the characteristic variations in the mobility of the driving transistors.
Next, at a time T5, the scanning line WS transitions to a low level side, and the sampling transistor 506 is thus turned OFF. As a result, the gate of the driving transistor 507 is electrically separated from the signal line SL and, at the same time, the drain current of the driving transistor 507 starts flowing through the luminescence element 508. After this, Vgs is maintained constant by the capacitor 509. The value of Vgs here is obtained by correcting the signal voltage Vsig using the threshold voltage Vth and the mobility β.
Lastly, at a time T6, the potential of the bias line BS is restored to the high potential from the low potential so as to allow for a next frame operation.
As described so far, the display device 500 disclosed in Patent Literature 1 prevents the variations in luminance caused due to the variations in the threshold voltage Vth and in the mobility β.